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The only thing you will see a difference in really is either steel or aluminum. Aluminum is obviously going to be lighter and probably give you just a slight more gain compared to steel.
I got a set of aluminum UD pullies from Blue Oval on ebay($80). The only difference i noticed was that it reved up easier. Many people say there a waste of money, but small gains add up and I like the idea of squeezing out every horse you can.
With the March Performance Power & Amp UD pulleys I have and with the stock water pump and the stock alternator, both over 185K on them, I've never had a problem with charging or over heating issues while being stuck in stop and go traffic for 1-1/2 hrs ever once and awhile with the A/C on and listening to the stereo, granted the stereo system is nothing huge, but I'm not worried about a charging problem either.